Technology for positioning mobile radio terminals using the signals received from one or more transmitters has been widely used for many years. Such systems include terrestrial networks of transmitters (e.g. Loran) and networks of satellites (e.g. GPS and Galileo) deployed specifically for the purpose of locating the receiver, as well as methods that use general-purpose radio networks such as cellular mobile telephone networks (e.g. WO-A-97-11384) or TV and radio transmitter networks. (e.g. EP-A-0303371).
Within a cellular mobile telephone network, for example, the position of the terminal may be based on the identity of the serving cell, augmented by information such as the time delay between the serving transmitter and terminal, the strengths of signals received from the serving and neighbouring transmitters, or angles of incidence of received signals. An improved position may be obtained using the observed time difference of arrival (OTDA) of signals received at the terminal from two or more transmission sources.
OTDA methods give good position accuracy using only the signals available within the cellular radio network. However, they require the precise transmission time offsets between transmitters to be determined in order to solve the positioning equations. This can be done using location measuring units (LMUs) having additional receivers. LMUs are placed at known locations so that their OTDA measurements can be converted directly into a network timing model (see for example WO-A-00-73813).
Alternatively a technique (see WO-A-00-73814) may be used in which measurements of signals from a number of geographically disparate transmitters at known positions made, for example, by two geographically disparate terminals at unknown positions, may be used to compute both the positions of the terminals and all the timing offsets between the measured transmitters, without the need for LMUs.
Satellite positioning systems, such as GPS, provide an accurate solution provided that the receiver can receive sufficient satellite signals. The satellite signals are related to a common time-base of a globally defined standard time, e.g. GPS Time or Universal Coordinated Time, UTC. For example, within GPS, each satellite in the constellation has a stable atomic clock whose time is continuously measured and compared with a single reference clock located on the ground. The time of each satellite clock is steered towards alignment with the reference clock and a three-parameter model derived which describes the difference in time between the two clocks. The three parameters are up-loaded to the satellite and broadcast by the satellite as the clock correction parameters. This has the effect, after making corrections based on the parameters, of aligning the satellite clock closely with the ground-based reference clock. Satellite positioning systems work well in situations where the receiver's antenna has clear sight of the sky, but they work poorly, or not at all, inside buildings or when the view of the sky is obscured. Another problem is that they take a long time to achieve a “first fix” from a cold start and they therefore work best when they are tracking the satellite signals continuously.
In attempts to overcome these problems various proposals have been made to provide ‘assistance’ to satellite positioning systems. For example, U.S. Pat. No. 5,663,735 discloses the provision of an additional radio signal, by means of a separate radio receiver, incorporating a standard time or frequency and using the standard time or frequency to determine the GPS time for the time of arrival of a data bit. In another example, (see WO-A-99-47943) a mobile cellular telephone network is adapted to receive GPS signals at a base transmitter station (BTS) to allow it to calculate the position of a mobile telephone.
In a further development (see US 2002-0168988 A) a GPS unit has a position determining system (PDE) which includes a reference signal receiver, typically part of a mobile communications system, and part of a reference signal received by the reference signal receiver is transmitted to the PDE to provide additional timing data which can be used to assist the GPS unit operation.
The sending of assistance data over a link has been known in the art for many years. One of the earliest examples was provided in 1986. White Sands Missile Range Interface Control Document disclosed position reporting over a two-way communications link which allowed for the transfer of either pseudo-range or computed location based on a geodetic coordinate reference frame as defined, from time to time, in WGS84 format. ICD GPS 150, dated 1986 and issued by the US government to potential bidders for the range applications joint programme, incorporated, inter alia, support for mobile GPS receivers through transmission of ephemerides, almanac and time information. Actual use of these data formats in support of mobile GPS receivers by means of two-way data-links has been made since 1986. A further example of assistance data is taught in U.S. Pat. No. 5,225,842 (Brown et al) filed 9 May 1991, granted 6 Jul. 1993, in which initialisation data is sent to a remote terminal (sensor) to enable the sensor to acquire and track the plurality of visible GPS satellites. The assistance data comprises initial estimates of the position of one or more objects and a satellite selection table.
Providing a satellite positioning system receiver with assistance data can enhance its performance. Furthermore, accurate timing assistance reduces the complexity of the associated chip sets. Assistance data may comprise all or some of three elements: a) satellite information, b) time aiding, and c) an estimate of the receiver's position.
Known in the art are methods by which the satellite information is provided by a server which is linked to one or more reference receivers that continually monitor the satellite signals in order to obtain the satellite information. In a GPS system, this information can also be obtained directly by the GPS receiver from the satellite signals whenever a satellite signal can be received. Time aiding may be obtained from network signals whose timings have previously been related to the satellite time base by network-based equipment. An estimate of a receiver's position may be obtained using a network positioning method, such as one based on OTDA. In all cases in the art, the assistance data is sent to the GPS receiver using a data channel provided by the mobile cellular network.
In our WO-A-00-73813 and WO-A-00-73814 (which are hereby incorporated by reference) we describe a communications system and method which constructs and maintains a timing model defining the timing relationships between transmitters in the cellular radio network. The system also computes the position of the receiver. By linking the timing of the signals from one or more transmitters of such a system to the GPS time base, this network timing model could be used to infer the timing of the signals transmitted by any transmitter in the network relative to the GPS time base and thereby provide timing assistance information to a GPS receiver. The position estimate may also be provided to the GPS receiver.
Other references describing assistance systems include U.S. Pat. No. 6,429,815 A, US 2002-0075942 A, US 2002-0068997 A, US 2002-0123352 A, WO-A-02-091630 and WO-A-01-33302.
In U.S. Pat. No. 6,445,927 (King et al.) there is described a method for computing the location of a base-station in a communications network, using measurements made by a mobile terminal of the time of arrival of communication signals from the base station with respect to GPS position information obtained from a GPS set carried within the terminal. A critical feature is that the terminal must be located in a minimum of three geographically disjoint locations before a solution can be found. The current invention is not concerned with location of the base station as that is information which is provided within the method.
In U.S. Pat. No. 6,603,978 (Carlsson et al.) there is provided a method and apparatus for providing time information assistance to a GPS receiver located in a mobile terminal via a wireless communication signal during active call sessions where the traffic and control channels are not necessarily synchronised. Unlike the current invention, this is achieved using location measurement units (LMUs) and GPS receivers associated with the base stations in the network, and time offsets are sent over the communications channel to the mobile terminal.
In a patent application published under US 2002/0168988 A1 (Younis), timing assistance is provided to a GPS set in a mobile terminal by using a reference signal (for example a public broadcast signal) which is received both in the terminal and in one or more receivers in the network. The terminal sends a snippet of the received reference signal to a network-based computing node, along with a request for GPS aiding information, where the time offset with respect to the reference signal is determined. This time offset is sent back to the terminal which uses the information to acquire GPS signals. As previously noted, the current invention does not compute any GPS time offsets in the network, and neither does it transmit such information over a communications link. Furthermore, the current invention does not transmit snippets of reference signals over a communications link.
In U.S. Pat. No. 6,084,547 a position calculation system is disclosed which uses historical information to enhance the accuracy of a radio position fix. The system latches an arriving signal to a clock counter for receiver array synchronisation but does not take advantage of or relate an arriving signal with a Universal Time, i.e. an unambiguous one with a defined start such as UTC, for example by utilising an independent oscillator in a receiver.
In summary, therefore, it is known that current systems for locating a mobile receiver using satellite positioning technology can be improved if they are supplied with accurate time aiding based on the timing of another signal, such as the signal received from the serving base station (the ‘downlink’) of a cellular mobile radio network. The time aiding is used by a satellite positioning receiver to reduce the range of time offsets over which it must search in order to detect a given satellite signal. The generation of accurate time aiding requires the time relationship between the satellite signals of the satellite positioning system (the satellite time base) and the downlink signals of the cellular network to be known. The timings can be measured and linked together using either LMUs installed at fixed known locations, or a network-based system such as described in WO-A-00-73813 and WO-A-00-73814. One or more GPS LMUs in the network can then be used to find the offsets between the network timings and the GPS time base. In such cases, time aiding is therefore only available when the mobile terminal has access to a properly equipped terrestrial radio network. Furthermore a significant amount of signalling and messaging is required both within the network, and between the network and the mobile terminal.
Calibrated time information, i.e. time information related accurately to a reference time such as GPS Time or UTC, can be used for many purposes. One of these, mentioned above, is to assist a GPS or other satellite positioning receiver to lock on to the signals from a particular satellite by reducing the uncertainty of the times of arrival of the signals, and hence reducing the range of time offsets over which the receiver must search in order to detect the signals. Another use of calibrated time information is in Very Long Baseline Interferometry where two radio astronomy receivers at either ends of the baseline (which may be thousands of km in length) must be synchronised with each other to within a time precision equal to the reciprocal of the receiver bandwidth (i.e. about 200 ns for a 5 MHz bandwidth).
The prior art outlined above relates generally to the provision of time aiding to a satellite positioning receiver. What is also needed is a method and apparatus within the terminal by which the time information is transferred from the communications receiver receiving the signals from the communications network to the satellite positioning receiver. This is not straightforward for several reasons.
First, the manner in which this is done in a commercial terminal must be cost-effective.
Second, there is always a processing delay between the reception of a signature in the network signal and the detection of the signal by the base-band processor in the terminal. This delay may be measured in milliseconds, and is therefore substantial when fine time aiding at the level of a few microseconds is needed. The delay may comprise both a fixed component, for example due to the execution time of a process or series of processes, and a random component which may be due to a waiting time for a required process to be triggered or for a process in execution to be completed.
Third, as the position of the mobile terminal changes, the time information has a transmission delay (time of flight component) which depends upon the relative location of the mobile terminal and the transmission source.
Fourth, it may be necessary in some systems to transfer the calibration of the received time information on the return path to the communications system for use by the system itself and by other users.
In U.S. Pat. No. 6,865,380 (EP-A-1229344) Syrjärinne teaches a method in which a pulse is directed from a communications module (e.g. a GSM receiver) along a ‘special hardware path’ (i.e. a wire link) to a user module (e.g. a GPS receiver) such that there are no substantial random delays. Whilst straightforward to implement, this invention fails to deal with the processing delay in the generation of the trigger pulse, so cannot be used for the provision of fine time aiding with microsecond accuracy. Nor does Syrjarinne deal with the problem of correcting the timing of the trigger pulse to compensate for motion of the communications or user module.
What is required is a method by which the failings of the prior art can be corrected. In many cases it is desirable to generate a ‘marker signal’, such as a trigger pulse, directly from the source of universal time such as that within a GPS receiver, rather than attempting to detect the arrival of the communications signal instantaneously.